This invention relates to a power supply apparatus for a resistance welder which as a current controlling function suitably adaptive to a resistance welder.
In power supply apparatus for resistance welders, the value of an AC voltage applied across the welding electrodes is varied by controlling the conducting periods of gate controlled rectifiers (hereinafter referred to as thyristors) connected in inverse-parallel relationship, in order to control the welding current flowing through a workpiece. However, when the thyristors are ignited or turned on and the welding current is conducted, a voltage drop develops across the impedance of the power source, causing the voltage wave form to distort with the result that the difference between the phases for turning on the two thyristors is deviated from the normal phase difference which is 180.degree.. Especially, when a great number of power supply apparatus for resistance welders are loaded, the deviation of the phase difference grows large.
In the event that the difference between phases for turning on the two thyristors is deviated from the 180.degree. normal phase difference, integrals of positive and negative halves of the voltage wave form become asymmetrical and hence the transformer core is subject to a polarized excitation. As a result, an overcurrent flows through the transformer to burn the same or break down the thyristors, also resulting in a defect that the welding current cannot reach a set value, leading to welding failures.